1. Field of the Invention
The present invention relates to a method and apparatus for image reading, and more particularly to a method and apparatus for image reading having a fixed reader capable of obtaining adequate white reference data used for a shading correction by protecting a white reference section read by the reader against dust or motes.
2. Discussion of the Related Art
Duplex image scanners have been developed capable of reading front and back sides of double-sided original documents with high quality at high speed. Such a duplex image scanner generally includes a flatbed unit and an automatic document feeder (ADF) unit.
The flatbed unit includes a contact glass and a first optical system. The first optical system moves back and forth in a sub-scanning direction beneath the contact glass. The scanner carries out a main scanning and a sub-scanning on an original document which has been set manually or transferred by the ADF onto the contact glass. The ADF unit is provided with an ADF reading glass to which the first optical system moves to read one side of the original document being conveyed. The ADF unit is also provided with a second optical system which is placed on a downstream side of a document conveying path and reads the other side of the document being conveyed.
In the image scanner, there arise problems such as fluctuations in light amount of a lamp or variations in sensitivity of photosensitive pixels in a photoelectric conversion element like a charge-coupled device (CCD). To overcome these problems, the scanner performs a shading correction on original image data by using the white reference data obtained by reading a white reference plate.
The duplex image scanner is provided with the first optical system in the flatbed unit and the second optical system in the ADF unit. The scanner is also provided with a first white reference plate and a second white reference plate used by the first and second optical systems, respectively. The first white reference plate is provided just beneath the contact glass in order to protect the plate against dust, paper powder, motes, and so on caused by motion of the first optical system. The first optical system is moved under the white reference plate to read a predetermined length of the white reference plate irradiated with a lamp. The first white reference plate is provided beneath the contact glass, where contact with the dust or motes is prevented. However, the first white reference plate is still susceptible to undesired substances caused by gas of the lamp or motion of the scanning members. To avoid these problems, a conventional duplex image scanner does not read only one line, but reads a plurality of lines over a length of several millimeters of the first white reference plate, and averages the data obtained to generate shading data for one line. The shading data is stored in a memory and used in the shading correction to the image data of the original document.
As for the second optical system which is fixed to the ADF, the second white reference plate is disposed directly under the second optical system on the document conveying path. When no document exists on a scanning line of the second optical system, the second optical system reads the second white reference plate to obtain shading data, as described in Japanese Patent Laid-Open Application Publication No. 2001-16412.
Since the second optical system is immovable, it reads one line of the second white reference plate once or more to obtain shading data.
When there are dust particles or motes on the scanning line of the second optical system, the shading data would be of no help to remove fluctuations in light amount or variations in sensitivity of photosensitive pixels in a CCD sensor, failing to satisfy its intended purpose. Using such shading data in the shading correction might cause undesired lines in the image being processed, thereby reducing quality of the image. In a case of a scanner with a resolution of 600 dpi, for example, even a small grain of dust may cause a problem to the image of a size equivalent to 0.0423 mm, a line width of the scanner.
Moreover, being disposed directly under the second optical system where the original document passes through, the scanning line of the second white reference plate is susceptible to paper powders that have fallen from the document, thereby often reducing quality of the image being processed.
Alternatively, some image reading apparatuses include a white platen roller. In order to suppress an adverse effect of dust or motes, the white platen roller rotates when the image reading apparatus reads shading data, as described in Japanese Patent Laid-Open Application Publication No. Hei 05-319613.
Even if the above technique is applied, however, it is still difficult to protect the white platen roller against dust or motes inevitably existing in the air. Inevitably, over the course of time, paper dust clings to the white platen roller, burning caused by the lamp discolors the roller, gas generated by the lamp causes the roller to deteriorate, and so on. When the second optical system reads data from such a roller, the resultant data may include stains or be of uneven density, thereby reducing the quality of the image.